1. Field of the Invention
This invention relates to methods for interfacing low voltage circuitry to high voltage circuitry, and in particular, to an interface circuit which operates from the low voltage power supply to detect differential signals which may have signal voltages that exceed the voltage of the low power supply.
2. Description of the Related Art
To reduce power consumption and to allow for increased integrated circuit densities, power supply voltages are being reduced. However, the voltage migration is not occurring simultaneously in all system components. Building a system using digital components powered by different supply voltages causes numerous interfacing problems. For example, transistors in the reduced voltage circuit may be subject to failure if forced to sustain the larger power supply voltage. Protecting these transistors with limiting diodes may prevent the transistors from detecting the desired signals, particularly in the case of differential signaling.
FIG. 1 shows a schematic of a differential amplifier. A current source 102 draws a current from two transistors 104 and 106 which are in parallel. Each transistor has a corresponding resistance 108 and 110 connected between the drain and the supply voltage. The current and the resistances are chosen so that whichever transistor receives the larger gate voltage carries most of the current and the corresponding resistor sustains a voltage drop nearly equal to that of the supply voltage. Hence, if the input voltage VIN is larger than the reference voltage VREF, transistor 106 conducts most of the current drawn by current source 102, and the output voltage VOUT is close to ground. If VIN is supplied from, say, a 2.5V circuit, and transistor 106 is designed using a 1.5V process, then the 2.5V drop between the gate and the drain will cause transistor failure.
An exemplary implementation of current source 102 is shown. Current passing through resistance 112 and transistor 114 self-biases transistor 114. As circuit conditions vary, the temperature coefficient of transistor 114 causes more or less current to be drawn, but this is compensated by the change in gate voltage that results. This provides an accurate bias to transistor 118 with resistance 116 to draw the desired current from transistors 104 and 106. Other current source and current mirror implementations are well known and may be used.
FIG. 2 shows a schematic of a differential amplifier 202 with protected inputs. Differential transistors 104 and 106 receive their input voltages via protection transistors 204 and 206 which have their gates tied to the supply voltage. Input voltages up to the supply voltage VCC minus the threshold voltage VTH are passed to the differential transistors, while voltages higher than that are blocked. Unfortunately, this circuit loses its differential behavior once the reference voltage exceeds VCC-VTH. For an exemplary power supply voltage of 1.5V, a reference voltage of 1.25 V (provided by a 2.5V circuit), would exceed the 1.0V maximum signal level that could be detected. The differential amplifier 202 is then unable to detect a high logic level at VIN.
A better method for interfacing a reduced voltage circuit to a circuit providing higher voltage signals is therefore desirable. The interface would provide for continued differential signal detection at voltage levels exceeding the reduced power supply voltage without requiring that a higher power supply voltage be provided.